System for downloading model files from Github

[justjake]

Packing even a single model into the application makes the app too big to release on Github. We could split the app itself into multiple files, but that's confusing to explain to end-users.

Instead, Gauss should be smart enough to download models from Github by itself into it's Application Support directory. That way, we can release a small application (<50mb ideally).

[justjake]

Uploaded model files to Github here: https://github.com/justjake/Gauss/releases/tag/models-v1.0.0